1. Field of the Invention
The present invention is directed to a power amplifier of the type having at least one switched output stage, a control unit which determines drive data for the output stage and for generating a data signal embodying the drive data, and a modulator, supplied with the drive data, which generates switching signals for the output stage dependent on the drive data. The power amplifier of the invention is particularly suited for use as a gradient amplifier of a magnetic resonance tomography apparatus.
2. Description of the Prior Art
German OS 38 22 990 discloses a number of embodiments of a power amplifier, particularly for the low-frequency and audio frequency range. In one of these embodiments the power amplifier includes a control circuit to which a number of switched output stages are each connected via respective control line. The control circuit in turn contains a pre-modulator that is connected by first transmission lines to a corresponding number of pulse duration modulators. The pulse duration modulators are connected to a common reference oscillator stage via second transmission lines and are connected to the switched output stages via the control lines.
In the specific use of a power amplifier in a magnetic resonance tomography apparatus, a magnetic field gradient is generated by a gradient coil that is connected to a gradient amplifier. The gradient amplifier delivers voltages on the order of magnitude from a few hundred up to a few thousand volts in order to produce an exactly regulated current in the gradient coil. This current fluctuates in a predetermined curve, for example between 0 and 300 A. Such gradient amplifiers with a switched output stage are well known, for example from German OS 40 24 160. The switched output stage typically has four switch elements connected to form an H-bridge. Respective switch signals are generated for driving each switch element. The switch signal communicates the binary information "switch element on" or "switch element off".
Amplifiers having a number of switched output stages that are connected in series at the output side are also known. Such gradient amplifiers are utilized for modern imaging methods wherein particularly high output voltages are required. German OS 43 04 517 discloses such an amplifier having two or-more output stages. Each output stage has two or four inputs at which switch signals from control units are supplied.
It is also known to employ a signal transmission link with an optical fiber or opto-coupler for the transmission of a switching signal from the control unit to the output stage. This is required in view of the high voltages and currents in the output stage. If there were a direct line connection between the control unit and the output stage, then noise pulses could be transmitted to the sensitive control unit, which is fashioned as a low-amplitude signal component.
There is the problem, however, that a signal transmission link suitable for employment in a gradient amplifier, particularly such a link having a light waveguide, is relatively complicated and expensive. Four such signal transmission links are already required given a gradient amplifier having a single output stage. Twenty transmission links are required for an amplifier having, for example, five output stages, each of which has a respective full bridge. This causes high costs. In the embodiment with opto-couplers, moreover, multi-pole lines having corresponding plugs and sockets are required. Due to the many cables or light waveguides, further, there is a substantial risk of mistake in manufacturing as well as in maintenance.